The present invention relates to methods of constructing vehicle bumper systems, and in particular relates to methods of constructing bumper systems for passenger vehicles incorporating a thermoformed energy absorber.
Modern bumper systems are designed to absorb a maximum of impact energy over a given stroke. At the same time, they are designed to minimize load spikes, and to distribute energy in a manner promoting uniform and predictable collapse upon undergoing a vehicle crash. Every millimeter of space is important to energy absorption, even spaces of 10 mm or less. Further, the individual components of an energy absorbing system must combine well with other energy absorbing components, e.g. metal tubular beams and non-tubular channels, injection-molded “honeycomb” energy absorbers, foam “block” energy absorbers, hydraulic shock absorbers, crush towers and mounts, and various combinations thereof. At the same time, light weight must be maintained. Also, it is desirable to maintain an ability to customize energy absorption at selected impact zones (e.g. at a corner of the vehicle or at a center impact, such as with a post impact). Concurrently, all components of a bumper system must be flexible, and able to conform to an aerodynamic sweeping curvature of a vehicle front.
Notably, thermoformed parts have not been used much on exterior bumper systems for modern passenger vehicles, since it is generally accepted in the bumper industry that energy absorbers must be relatively deep parts (such as about 40 mm or more deep) and include significant wall thickness (e.g. 3 mm or greater wall thickness) in order to provide a good crush stroke and energy absorption during impact. Further, most injection-molded energy absorbers made of solid polymer are relatively complex parts with undulating surfaces, varied wall thicknesses, and different wall spacings to provide optimal energy absorption in different regions of the energy absorbers. This is directly in opposition to thermoformed parts, which are basically limited to relatively short depths, relatively thin wall thicknesses (or at least reduced wall thicknesses in stretched areas), and no undercut/blind surfaces. Thus, for years, original equipment manufacturers of passenger vehicles have avoided using thermoformed parts, despite the fact that thermoformed molds generally cost less, require shorter lead times, provide faster cycle times, have lower thermal energy use, generate less waste, and are more environmentally friendly processes. Skilled artisans in bumper design have apparently not fully realized the unexpected added benefits that thermoformed parts can offer when combined with other energy absorbing systems and components.
Modern vehicle bumper systems often incorporate an injection-molded polymeric energy absorber on a face of a metal reinforcement beam, and thereafter cover the energy absorber and beam with a fascia. The cost of energy absorbers and also the cost of tooling and machine time can be significant. It is desirable to use a single energy absorber on different vehicle models, even when a shape of the fascia for the vehicles is slightly different in order to achieve higher part volumes. Sometimes a single energy absorber can be designed to work for two different fascia shapes, but this usually results in some undesirable gap(s) between the energy absorber and the fascia that it is supporting. This can result in sagging and drooping of the fascia in the area of the gaps due to lack of support, potentially leading to consumer complaints and/or the appearance of poor quality in the bumper system. It is desirable to provide a system where similar bumper systems can use the same energy absorber, even when a shape of the reinforcement beam and the associated fascia are slightly different in shape.
Another requirement of bumper systems is that they be able to provide a desired optimal energy absorption profile (i.e. energy absorption force vs displacement upon impact). As vehicles become smaller, the “package” space available for the bumper system also becomes smaller. It is important that every portion of the bumper stroke be used to absorb impact energy, even when the space is as little as 10 mm to 20 mm. Also, it is important that the energy absorber crush flat when impacted, so that it does not take up unnecessary space as the bumper system nears an end of its crush stroke. It is also important that the energy absorber be tunable to “adjust” energy absorption profile, which can be done in thermoformed parts by changing materials or by changing a thickness of the sheet being thermoformed.
Accordingly, a bumper system is desired having the aforementioned advantages and solving the aforementioned problems.